Microbial processes in seawater and sediments can convert inorganic mercury into toxic methylmercury (MeHg). The objective of our research is to identify relationship between microbial abundance and mercury species in the Adriatic seawater. Samplings were performed aboard the research vessel Bios Dva from February 2014 to December 2015 in the Central Adriatic Sea. Research was constrained to five sampling stations in transect from the coastal Kastela Bay to the open sea island of Vis. Seawater was sampled for the determination of total mercury (THg), dissolved gaseous mercury (DGM), methylmercury (MeHg) and microbial species in Adriatic coastal and open waters. Plankton samples for the determination of THg and MeHg was collected using vertical towing nets.

The highest THg concentrations in seawater (4.58-27.8 pmol/L) are found in the Kastela Bay, which was affected by previous contamination from chlor-alkali factory. THg values are the lowest in the pristine environment of the island of Vis (0.69-5.48 pmol/L). DGM always shows lower values in the pristine environment (0.11-1.23 pmol/L) than in contaminated coastal stations (0.16-1.75 pmol/L). MeHg concentrations range from 0.05-0.17 pmol/L for all stations, with the highest values found in the Kastela Bay. The average percentage of THg present as MeHg is low (2.23%) indicating deficiency of mercury methylation or high MeHg demethylation rates. Mercury fractions (THg, DGM and MeHg) are significantly correlated with the total bacterial abundance, number of picoeukaryotes, and the number of cyanobacteria from the genus Prochlorococcus (Spearman Rank Order Correlation, P < 0.01). In addition, THg and DGM are correlated with the number of cyanobacteria from the genus Synechococcus (Spearman Rank Order Correlation, P < 0.001). These correlations indicate association of mercury species with the smallest members of the marine microbial community, and possible mercury transformations in the water column by the autotrophic picoplankton.

The highest average THg concentrations in plankton are found in the Kastela Bay (1.69 and 0.91 nmol/g dry weight for 53 and 200 µm fraction, respectively), while the lowest values are found at the Split Channel station (0.29 and 0.27 nmol/g dry weight for 53 and 200 µm fraction, respectively). The MeHg concentration are significantly higher (t-test, P < 0.001) in the 200 µm fraction, compared to 53 µm fraction (on average 59.4 and 22.6 pmol/g dry weight, respectively). These results show biodilution effect of THg concentrations in plankton, which is best observed in the Kastela Bay. On the contrary, MeHg bioaccumulation along trophic levels is demonstrated through significantly higher MeHg concentrations and MeHg percentage in 200 µm fraction, compared to 53 µm fraction.